Tolkien himself did this, don't forget. He originally wrote the Hobbit as a stand alone book, then went back and changed it so that it fitted in with LOTR. He then went further, in The Silmarillion and Unfinished Tales, to explain how the events in the Hobbit came about, and explained what went on to make them happen.

Whilst it is clear that the story originally started out as a single tale, it's very, very clear that Tolkien saw this tale as part of the whole picture, one piece in the jigsaw puzzle. To my mind, it's absolutely fine to replicate that in the films. Mawr yw ein braint i berthyn i'r gwm Llynfi